Chop the watermelon. Add the melon and mint to a cocktail shaker. Muddle (gently mash) them with a cocktail muddler or wooden spoon to release the juices. (If you’re using watermelon juice, simply muddle the mint first.)
Add the vodka, Cointreau, lime juice and simple syrup. Fill with ice and shake until cold.
Strain the drink into a cocktail glass. Garnish with a small watermelon wedge, if desired.